This invention relates to a process for producing a colloidal solution of a curable resin containing a polyfunctional maleimide component. This invention also relates to a colloidal solution of a curable resin which can be used as a coating, as an impregnating agent for a coil, such as an electrical coil, or for preparing prepreg.
In the prior art, since cured products obtained by curing curable resin compositions containing a polyfunctional meleimide component have heat resistance, use of the curable resin compositions as an insulating material has been considered. However, curable resin compositions containing a polyfunctional maleimide component are poor soluble in most organic solvents. Limited high boiling point solvents, such as N-methyl-2-pyrrolidone (boiling point of 204.degree. C.) or dimethylformamide (boiling point of 153.degree. C.) have been used as a solvent for dissolving such curable resin compositions containing a polyfunctional maleimide component. Low boiling point solvents may be used together with high boiling point solvents, or may be used as a resin solution having a concentration of as low as 30%.
For example, use of high boiling point solvents as a solvent for curable resin composition are disclosed in Japanese Patent Publication (Kokoku) Nos. 24547/1977, 47796/1977, 5920/1978 6040/1978, 24640/1978 and 16976/1979.
When a resin solution containing a high boiling point solvent is used as a coating, as an impregnating agent for electrical coil or for preparing prepreg, followed by drying the resin solution at room or high temperature, the high boiling point solvent used in the resin solution can not be completely removed from the finished product by such drying step. In other words, even though the finished product containing the high boiling point solvent is dried at an elevated temperature or room temperature, some of the high boiling point solvent is likely to remain in the product. When any high boiling point solvent remains in the product, the product tends to blister during processing of the product at an elevated temperature. In addition, the product containing the solvent has poor mechanical properties.
As mentioned above, when a low boiling point solvent is used for dissolving curable resin, a resin solution of only low concentration can be obtained, because of low solubility of the resin in the low boiling point solvent. However when the resin solution having low concentration of resins is used as for coating, impregnating a coil, such as an electrical coil or for preparing prepreg in one stage, amount of resin coated or impregnated is not sufficient, so such solution is not practical.